1. Field of the Invention
The present invention relates to image forming state adjusting systems, exposure methods and exposure apparatus, and programs and information storage mediums, and more particularly, to an image forming state adjusting system that optimizes the image forming state of a pattern by a projection optical system used in an exposure apparatus, an exposure method and an exposure apparatus that can actually achieve the optimization of the image forming state, and a program that makes a computer execute optimizing the image forming state of the pattern in the exposure apparatus and an information storage medium in which the program is stored.
2. Description of the Related Art
Conventionally, when manufacturing devices such as semiconductors, liquid crystal displays, or the like in a photolithographic process, projection exposure apparatus such as a reduction projection exposure apparatus based on a step-and-repeat method (a so-called stepper) and a scanning projection exposure apparatus based on a step-and-scan method (a so-called scanning stepper) have been used. Such exposure apparatus transfer a pattern of a photomask or a reticle (hereinafter generally referred to as a ‘reticle’) onto a substrate such as a wafer or a glass plate on which a photosensitive agent such as a photoresist is coated, via a projection optical system.
When manufacturing devices such as a semiconductor, because different circuit patterns have to be formed overlaid on the substrate in multiple layers, it is important to accurately overlay the pattern formed on the reticle and the pattern already formed on each shot area on the wafer. In order to perform such overlay with good accuracy, it is absolutely necessary for the image forming quality of the projection optical system to be adjusted to a desired state (for example, a magnification error of a transferred image of a reticle pattern to a shot area (pattern) on a substrate is corrected). Even in the case of transferring a reticle pattern of the first layer to each shot area on the substrate, the image forming quality of the projection optical system is preferably adjusted so that the reticle pattern of the second layer and onward is transferred with good precision on each shot area.
As a premise for adjusting the image forming quality (one of the optical properties) of the projection optical system, the image forming quality has to be accurately measured (or detected). As a measuring method of the image forming quality, a method in which the image forming quality, or to be more specific, Seidel's five aberrations (distortion, spherical aberration, astigmatism, field curvature and coma) is calculated (hereinafter referred to as ‘exposing method’), is mainly used. In this exposing method, exposure is performed using a measurement reticle on which a predetermined measurement pattern is formed, then the transferred image obtained by developing the substrate where the image of the measurement pattern is projected and formed, such as the resist image, is measured, and then the image forming quality is calculated based on the measurement results. Besides such a method, a method in which the above-mentioned five aberrations are calculated without actually performing exposure (hereinafter referred to as an ‘aerial image measurement method’) is also used. In this method, a measurement reticle is illuminated with an illumination light, and aerial images (projected images) of measurement patterns formed by the projection optical system are measured, and then the above five aberrations calculated based on the measurement results.
However, with the above exposing method or aerial image measurement method, in order to obtain all five aberrations, the measurement has to be repeated separately, using the appropriate pattern for each measurement. Furthermore, depending on the type and amount of the aberration to be measured, the order in which the measurement is performed has to be considered, in order to accurately adjust the projection optical system. For example, when coma is large, the image of the pattern is not resolved, therefore, when aberration such as distortion, spherical aberration, astigmatism are measured in this state, accurate data cannot be obtained. Accordingly, distortion or the like has to be measured, after the coma has been reduced to a certain level.
In addition, due to higher integration of semiconductor devices or the like in recent years, circuit patterns are becoming finer, which makes correction of only the Seidel's five aberrations insufficient, and requirements are pressing for an overall adjustment, including the higher order of aberrations, in the image forming quality of the projection optical system. In order to perform such overall adjustment in the image forming quality, a light-ray-trace computation has to be performed using data (such as curvature, refractive index, and thickness) of individual lens elements composing the projection optical system, to identify the lens element that requires adjustment and to calculate its adjustment amount.
However, because data of individual lens elements are confidential for the exposure apparatus maker, it is usually difficult for a service technician repairing or adjusting the exposure apparatus or a user to obtain such data. In addition, since the light-ray-trace computation requires an enormous amount of time, it is not realistic for the service technician to perform the computation on site.
In addition, to adjust the image forming quality or the image forming state of the projection optical system, for example, an image forming quality adjustment mechanism or the like that adjusts the position and the inclination of the optical elements such as lens elements making up the projection optical system is used. However, the image forming quality changes depending on exposure conditions such as illumination conditions (such as illumination σ), N.A. (numerical aperture) of the projection optical system, and the pattern to be used. Accordingly, the optimal adjustment position of each optical element by the image forming quality adjustment mechanism under a certain exposure condition may not necessarily be the optimal adjustment position under other exposure conditions.
Against such background, a new system was expected that could smoothly calculate adjustment position of each optical element by the image forming quality adjustment mechanism that brings out the optimal image forming quality under any exposure condition, such as the combination of N.A. of the projection optical system, illumination σ, and the subject pattern.